1. Currently composing for 2-3 clients (Games, therefore more longterm stuff currently)
2. Finding out what the client wants, translating what he says into composer speak for my notes
3. For me, negotiating terms

Apart from that - the hardest part is to find work with has a budget, given I currently still am in the indie scene and can't afford a lot of cash to go networking outside my country (switzerland) which is otherwise a dead place - be it games or film/tv

I wrote a long and involved set of cues for a collegiate (abysmally independent) film director. It was a rent's-paid-for-six-months commission and I put my heart into it. The hardest part was a director who loved a certain sound too much, and was obviously displeased when he didn't hear that amongst my cues.

It can suck having to basically ape another's work, but it was for pay. I have plenty of time for my artsy fartsy crap, anyway.

A couple months ago was the last film I worked on. I haven't had much luck in the commercial world which is fine, film scoring is where I'm most comfortable.

Echoing what others have said, negotiation. A lot of directors want you to start right away when terms haven't even been agreed to and honestly, if that's not done and they're not going to work with me I'll probably walk. It's a waste of my time and theirs. Having things in writing is so important.

The longest part on this go around was when one of the producers pulled out and suddenly there was no money for an editor. The director had cut a few scenes and we wanted to make sure we had cues timed right when suddenly there were no new cuts being produced. I was told to sit tight for awhile while money was being secured. In the end I finished with the cut we had and entrusted it to the editors :/

Currently writing an album's worth of orchestral music for a client. This is the second album I've done for him. His name is attached but his actual contribution to the music is extremely minimal. I guess you could say I'm ghost writing his half. Either way the pay is good, and there is a lot of creative freedom. The hard part is organizing my ideas. When a project is this big in scope, it can be a challenge. Also, finding ways to incorporate his more rudimentary ideas in with mine is not easy.

Finding a way through the hierarchy of a project... having a director, a producer, an editorial department, sometimes even an end client company with a similar structure... and all of them want different things and for no money. Especially in advertising agencies, a melting pot of wannabe alpha leaders. None of them can tell you what they want (as their decision could be shown as a mistake at the end) but everyone knows exactly what they don't want. In most cases the work you just did.